Rapa Nui subtropical broadleaf forests

Rapa Nui, also known as Easter Island, is one of the more famous islands on Earth. It is also the most geographically isolated and forms the eastern geographic and cultural boundary of Polynesia. Although paleobotanical studies suggest that the island was originally covered by palm and broadleaf forest, the island is now completely grass-covered, except for a few isolated stands of ornamental trees and shrubs. Rapa nui is one of the most isolated islands in the world and thus has an interesting biogeographic and ecological history.

Scientific Code

(OC0111)

Ecoregion Category

Oceania

Size

100 square miles

Status

Critical/Endangered

Habitats

Description Location and General DescriptionLocated in the South Pacific at about 27ºS latitude, Rapa Nui is 3,700 km west of Chile and about 2,200 km east of Pitcairn Island. It is the most remote inhabited spot on Earth. The climate is subtropical, with southeast trade winds from October to April. Annual rainfall averages 1,250 mm, with a rainy season occurring in the winter. Average temperatures range from 19ºC in winter to 24ºC in summer.

Rapa Nui is the youngest and westernmost of a chain of submarine volcanoes that probably formed as a volcanic hotspot track on the Nazca Tectonic Plate, located on the eastern side of the East Pacific Rise. The island is roughly triangular with an area of 166 km2 and a maximum elevation of 600 m. It is composed of three main volcanic summits, Rano Kau, Poike, and Terevaka, as well as several smaller vents. The oldest is Poike, which erupted in two episodes, one 9 million years ago and the other 2.5 million years ago. Lavas from Rano Kau are as old as 940,000 years, and lavas from Terevaka are the youngest with flows as young as 300,000 years. The last volcanic eruptions on Rapa Nui occurred about 13,000 years ago. The famous statues, or moai, of Rapa Nui were carved from the island’s yellowish-gray basaltic tuff, and their red topknots were carved from basaltic spatter.

Sala-y-Gomez is a small reef 415 km to the northeast of Rapa Nui. It is only 300 m long at low tide, and shrinks to a mere 70 m at high tide. Constantly subjected to salt spray, only four species of terrestrial plants grow here. A small depression sometimes contains fresh water. Large populations of seabirds use the tiny island for breeding. The terrestrial portion of the island is designated as a nature sanctuary.

The topography and the biota of Rapa Nui have been shaped by repeated climatic changes over the last 38,000 years, as well as by the most recent volcanic eruptions.

Compared to other subtropical volcanic islands, Rapa Nui is floristically species-poor, owing to its extreme isolation, and the fact that it has never been connected to any continental landmass. Carlquist (1967) estimated that more than 70 percent of the island’s indigenous plants were introduced by birds. Recent estimates put the total number of extant plant species at about 150, with 45 considered indigenous, including 3 endemic species of grass (Paine 1991). The precise nature of the flora is uncertain as studies are ongoing and many authorities disagree on the identification and classification of many plants.

The arrival of the first humans in the Fifth Century AD, and the subsequent arrival of Europeans, profoundly altered the flora so that little of the original vegetation is left.

Today, the island is almost completely grass-covered, except for a few isolated stands of ornamental trees and shrubs. In fact, it was almost treeless when first visited by Europeans in the early eighteenth century. At that time, the only trees were found on the steep inner slopes of the Rano Rakau crater (Fosberg 1998).

However, recent paleobotanical studies of fossil pollen and volcanic tree molds indicate that, prior to the arrival of the first Polynesian settlers, the island hosted an extensive array of trees, shrubs, ferns, and grasses. Vegetation communities were distributed in zones at different elevations, especially on the flanks of the volcanoes Rano Aroi and Rano Raraku. Over time, these communities varied in composition and structure, due to the climatic fluctuations occurring during the final phases of the Pleistocene and the beginning of the Holocene. Some of the tree species that dominated these ancient forests included a now-extinct palm, related to the Chilean palm, Jubaea chilensis, and the toromiro (Sophora toromiro). Shrubs included the hau hau (Triumfetta semitriloba), which is still present, and Coprosoma spp., which have since disappeared from the island (Rauch et al. 1996, Mueller-Dombois & Fosberg 1998).

Sophora toromiro was the only species of tree known on the island in historic times; it is now extinct in the wild. During botanical investigations between 1920 and 1953, Skottsberg found one remaining individual of S. toromiro, which has since died (Fosberg 1998). Most of the grasslands are now covered by introduced Stipa spp., Nasella spp., Sporobulus indicus and native Cynodon dactilon.

The long-extinct Jubaea palm is thought to have been the principal wood used to transport the immense stone statues of Rapa Nui. Recent carbon-14 dating, using mass spectrometer analysis, shows that Jubaea was present on the island as recently as the middle of the Seventeenth Century (Arnold et al. 1990). It has been hypothesized that over-exploitation by the island’s early inhabitants caused the extinction of this once dominant palm. However, Arnold et al. (1990) speculate that climatic change may have been at least partly responsible.

The bottom of Rano Rakau crater is covered with thick stands of tall bulrushes, Scirpus tautara, which may have been brought to the island by prehistoric voyagers from South America (Fosberg 1998).

The fauna of Rapa Nui includes four species of terrestrial birds, and three species of marine birds. There are no mammals except introduced rodents and carnivores. Two terrestrial reptiles, Lepidodactylus lugubris and Ablepharus boutoui poecilopleurus, are found on the island. There are several micro-lepidoptera, most are widespread in the Indo-Australian tropics and all are believed to have dispersed from the west. Although three of the species on Rapa Nui are cosmopolitan in nature, there appears to be no connection with South America. One species, Asymphorodes trichograma, is restricted to Rapa Nui and the Marquesas (Holloway 1990).

Biodiversity FeaturesRapa Nui is the most isolated island in the Pacific and that makes it extremely valuable for studying the biogeographical distribution of species. It is currently the focus of numerous paleobotanical studies that are rapidly changing the understanding of its native flora.

Ferns are one of the few higher plants that can be considered indigenous to Rapa Nui (Aldén 1990). Just 4 of the 15 reported species are endemic: Doodia paschalis, Polystichum fuentesii, Elaphoglossum skottsbergii, and Thelypteris espinosae (Rauch 1996).

Triumfetta semitriloba, a woody shrub, was once thought to have been introduced to the island because it is an important textile plant. However, pollen analyses have shown that it existed on Rapa Nui at least 35,000 years ago. For some time, it was thought to be extirpated from the island, but at least four individuals were found in 1988 (Aldén 1990).

Current StatusThe first Polynesians settlers reached Rapa Nui in the Fifth Century AD. It is uncertain when extensive deforestation began, though the island was basically deforested by the time the first Europeans arrived in 1722. Many exotic species have been introduced to the island. In recent studies, Zizka (1991) identified a total of 46 indigenous plant species of which 9 were endemic and 166 introduced. Alien plants and animals make conservation and restoration of remnant natural communities and species challenging. For example, grazing by horses and other herbivores must be controlled before native species can regenerate. Recent efforts to reintroduce the toromiro have been undertaken by Kew Gardens.

Types and Severity of ThreatsAlthough the 68 km2 Rapa Nui National Park was established as a protected area in 1935, islanders do not recognize the authority of the Chilean government and commonly ignore park regulations (Paine 1991). Alien plants, introduced grazers, and fires represent a major threat to remnant native plant communities and populations. Other current environmental problems include damage from archaeological investigations, erosion, and tourist-caused damage. Chile has recently announced plans to increase the pace of development on Rapa Nui.

Justification of Ecoregion DelineationEaster Island is arguably the most isolated island in the world. Before the arrival of Polynesians, the island was forested with trees (Sophoro toromiro, Triumfetta sp.) and a palm (Jubaea disperta)(Mueller-Dumbois & Fosberg 1998, Steadman 1995). Steadman (1995) identified the remains of at least six species of land birds from four families in prehistoric sediments. On the basis of its unique prehistoric fauna and extreme isolation, Rapa Nui has been delineated as a distinct ecoregion.

ReferencesAldén, B. 1990. Wild and introduced plants on Easter Island: A report on some species noted in February 1998. Pages 209-216 in H.M. Esen-Baur, editor. State and Perspectives of Scientific Research in Easter Island Culture. Courier Forschungsinstitut Senckenberg, Frankfurt, Germany.

Paine, J.R. 1991. IUCN Directory of Protected Areas in Oceania. World Conservation Monitoring Centre in collaboration with IUCN Commission on National Parks and Protected Areas and the South Pacific Regional Environment Programme.